In most of a conventional rewritable optical disk system, a guiding groove for guiding a focussed light is provided on an optical disk, a focussed light is guided along the guiding groove and information is recorded and reproduced on the optical disk.
The surface of the rewritable optical disk system is provided with two track, one is a groove track and the other is a land track, where the aforementioned guiding groove serves as the groove track and the land track is positioned between the adjacent groove tracks. An information-recording track is formed along the center line of either the land track on the groove track. The objective lens mounted on an optical head focuses a laser light on the surface of the optical disk to form a spot thereon. The objective lens is driven in both the focussing and tracking directions by a servo control system in order to follow vertical deviation and eccentricity of the optical disk. In order to achieve stable servo characteristic, the optical disk system must discriminate whether the spot is on the land or groove track. Since the widths of the land and groove tracks are different from each other, the total reflected light from the optical disk varies in a sinusoidal fashion around a certain constant, as the spot moves in the tracking direction. The rewritable optical disk system can discriminate whether the spot is on the land or groove track in reference to the total reflected light. However, if the widths of the land and groove tracks are equal to each other, the total reflected light becomes independent of the position of the spot and the aforementioned method is not successful.
Viewed from the another angle, if the rewritable optical disk system, which can discriminate whether the spot is on the land or groove track in case that the widths of both the track are the same, both the land and groove tracks can be used as the information recording tracks and optical data stored in the rewritable disk can be doubled.